Particulate filters are capable of filtering particulate exhaust gas constituents, particularly soot particles, out of the exhaust gas of internal combustion engines, thus preventing their emission into the atmosphere. In principle, it is possible for this purpose to use either surface filters or depth filters. Surface filters consist typically of ceramic materials, for example silicon carbide, cordierite, aluminum titanate or mullite. These filters achieve filtration levels of more than 95%. Alternatively to the surface filters, it is also possible to use open structures for deposition of soot particles. These open structures are primarily ceramic foams or filters composed of metallic wire braids. The filtration efficiency of these open filter systems is much lower than that of typical surface filters (<70%).
The actual challenge in the operation of a particulate filter in the exhaust gas of an internal combustion engine is, however, not the filtration of the soot particles but the periodic regeneration of the filters used. Since the temperatures of more than 550° C. required to ignite and combust the soot with oxygen in modern passenger vehicle diesel engines can typically only be achieved in full-load operation, additional measures are absolutely necessary for oxidation of the filtered soot particles, in order to prevent blockage of the filter. In general, a distinction is drawn here between passive and active heating measures. In the case of active measures, the temperature of the particulate filter is raised, for example, by using an electrical heater (for example glow plugs or else microwave heaters) or a burner operated with fuel. Such active measures are always associated with an increased fuel demand. For this reason, in many systems available on the market, the use of passive measures is preferred. In the case of passive systems, the use of catalysts lowers the ignition temperature required to combust the soot particles. This can be achieved through two different concepts. One concept is the use of organometallic fuel additives, for example cerium compounds and iron compounds, which are combusted with the fuel and become intercalated in the soot layer, finely distributed in the form of metal clusters, as a homogeneous catalyst. One alternative to the additive-based systems is the coating of the particulate filters with a catalytically active material.
Since the lowering of the soot ignition temperature by catalytic measures is generally insufficient to ensure full regeneration of the filter in all states of operation of the engine, the current practice is typically to employ a combination of passive and active measures. The combination of the particulate filter with an oxidation catalyst arranged upstream has been found to be particularly useful. As a result of injection of additional fuel in combination with other engine-related measures (for example partial throttling), unburnt fuel and carbon monoxide reach the diesel oxidation catalyst and are converted there catalytically to carbon dioxide and water. The heat of reaction released heats up the exhaust gas and hence also the downstream particulate filter. In combination with a lowering of the soot ignition temperature by means of a catalytic coating of the filter or else through the use of fuel additives, the injection required can be reduced and the filter can be regenerated virtually at any operating point within the characteristic map of the engine.
In the first generation of exhaust gas aftertreatment systems comprising catalytically coated particulate filters, the filters were usually arranged after one or two upstream oxidation catalysts in the underbody of the motor vehicle. In new exhaust gas aftertreatment systems, the filters, in contrast, are installed as close as possible downstream of the engine. Owing to the restricted construction space and to reduce costs, the oxidation catalyst in these cases is applied partially or completely to the filter. Such a filter arranged close to the engine must, to comply with the legal limits for carbon monoxide (CO) and hydrocarbons (HC) over the required distance traveled, possess a correspondingly high oxidation potential. In addition, it must also be capable over the entire run time of converting the hydrocarbons injected during an active filter regeneration, in order thus to generate the exothermicity required to achieve the soot ignition temperature. Furthermore, the catalytically active coating for use of such a filter close to the engine must have a high thermal stability.
To date, virtually exclusively filter coatings comprising platinum have been used for diesel passenger vehicles. Coatings comprising platinum and palladium have likewise become known (DE 102004040549 A1). Coatings comprising platinum and palladium are notable for a very good thermal stability, but possess a lower fresh activity than catalytic coatings comprising platinum alone. The principle of platinum- and palladium-containing coatings was first described some time ago. Owing to the significantly lower tolerance of Pt/Pd coatings toward poisoning by sulfur and the associated decrease in the catalytic activity, the use of palladium in exhaust gas catalytic converters for diesel passenger vehicles was prevented for a long time. Since, however, sulfurization is a poisoning which is reversible at high temperatures, the noble metal sites of the catalytic coating are simultaneously desulfated during the regenerations in the case of periodically actively regenerating systems, for example in the case of diesel particulate filters. This re-establishes the original oxidation activity.
WO 02/26379 A1 describes, among other things, a soot filter which comprises two catalyst layers one on top of another. The first layer is present on the inlet channels of the filter and comprises components for oxidizing carbon monoxide and hydrocarbons. These components consist of support materials with platinum group metals deposited thereon, the support materials being selected from the group consisting of aluminum oxide, silicon oxide, titanium oxide, zirconium oxide and zeolite, and the platinum group metals being selected from platinum, palladium and rhodium. The second layer is applied to the first layer and comprises components for lowering the ignition temperature of soot, more particularly at least one oxygen-storing compound and at least one platinum group metal.